Methods and Devices for Transmission Line Analysis

ABSTRACT

Improved diagnostics of transmission line noise are enabled by adapting DSL equipment to make measurements of quiet line noise also in the transmit bands, so that noise can be measured at both ends for the same frequency or frequency bands. 
     An estimate of the point where noise enters the line, as well as an estimate of the noise power at that point can be made from the relationship between the so measured noise levels at both ends of the line.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/148,550, filed on Sep. 23, 2011, entitled “Methods and Devices forTransmission Line Analysis”, which is a 371 of International ApplicationNo. PCT/SE2009/050474, filed on Apr. 30, 2009, International PublicationNo. WO/2010/093300, published on Aug. 19, 2010, which is related to, andclaims priority from, U.S. Provisional Patent Application No.61/151,714, filed on Feb. 11, 2009, the disclosure of which isincorporated here by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of transmission line analysis

BACKGROUND

DSL (Digital Subscriber Line) is a widespread technology for datacommunication over existing telecommunications transmission lines.

ADSL2+, described in ITU-T standard G.992.5 and G.992.3 and VDSL2,described in ITU-standard G.993.2 are examples of such technology. ITU-Tstandard G.997.1 describes physical layer operation and maintenancefunctions for several DSL standards.

DSL communications equipment is often adapted to perform variousoperation and maintenance functions, for example to aid in diagnosingcommunication problems

ITU-T standards G993.2 and G997.1 describe inter alia that VDSL2equipment can carry out measurements of quiet line noise (QLN) in theequipment's receive bands. This noise affects the signal to noise ratio(SNR) and hence the achievable data transmission rate and thereforethere is a desire to measure this parameter, e.g. for diagnosticpurposes. If the transfer function, H, of the line is also measured, thetransmit power needed to achieve a particular data rate may becalculated.

In general, there is a desire to provide improved transmission linediagnostics, for example to provide improved diagnostics of noiseproblems.

SUMMARY OF THE INVENTION

It is an object of the present invention to enable improved diagnosticsof noise problems, and a further object to provide such diagnostics.

Existing DSL equipment (DSLAMs and CPEs) generally are able to make somenoise level measurements. However, this capability has been focused onmaking noise measurements in the receive bands, since it is whenreceiving that noise may be a problem. When transmitting, noise is not aproblem, so there has been no interest in measuring noise in thetransmit bands.

Hence, noise is measured in different bands at the customer and centraloffice ends of the line respectively.

However, if noise is measured also in the transmit bands (which are theopposite end's receive bands), so that noise is measured at both endsfor the same frequencies, it becomes possible to estimate the pointalong the line where noise enters the line.

It also becomes possible to estimate the noise level at the point wherenoise enters the line.

Thus, when excessive noise in the receive band is a problem at an end ofthe line, a position where the noise enters the line and the noise levelat that point can be estimated by making such measurements and relatingthem to each other. Even though at any given end of the line it is onlythe receive band noise that is a problem, a noise measurement at theother end at the same frequency or frequency band (i.e. in a transmitband) nevertheless provides important diagnostic information. Hence,improved diagnostics of noise problems are enabled when suchmeasurements are possible.

A DSL communications equipment according to the invention therefore isadapted to measure quiet line noise, QLN in at least one of its transmitbands. (Quiet line noise is the noise that is present when the line isnot used for transmission). The adaption may be for measurement in apart of or the whole of one or more transmit bands, and the equipmentmay be e.g. a CPE (Customer Premises Equipment) or a DSLAM (DigitalSubscriber Line Access module). For example, measurement can be made onall frequencies normally used for reception and transmission of data.The equipment may conform to a particular DSL standard, such as ADSL2+or VDSL2.

Corresponding methods that make the measurements that the DSL equipmentis adapted for are also applicable. Said equipment and methods enableimproved diagnostics.

In other embodiments, improved diagnostics are provided.

In a method for estimating a position where noise enters a line, ameasurement result is received via the line from DSL equipment at thefar end of the line, and another measurement result is received from DSLequipment at the near end of the line. Then, a position is estimated independence of the relationship between the measurement results.Measurement was for the same frequency or frequency band for theequipment at both ends, and the frequency or frequency band falls withina transmit band of at least one of them.

There is also a corresponding device having means for carrying out thesteps of the method.

In another method a CPE measures a first noise level at its end of aline and communicates it to a DSLAM at the opposite end. The DSLAMmeasures a second noise level at the same frequency or frequency band atits end of the line. The frequency or frequency band falls within thetransmit band of at least one of the DSLAM or the CPE. Then, a positionwhere noise enters the line is estimated in dependence of therelationship between the first and second noise levels.

It is an advantage of the invention that improved noise diagnostics areenabled.

Another advantage is that improved noise diagnostics may be provided.

An advantage of improved noise diagnostics is that it becomes easier toidentify and rectify the causes of excessive line noise.

It is an advantage that the position where noise enters the line can beestimated, as this may significantly reduce costs for travel and fieldwork. In particular, if an underground cable needs to be inspected,pinpointing digging operations can save a lot of money.

An advantage of estimating the level of the noise at the point where itenters the line is that it may provide clues to identifying the noisesource.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic picture of noise ingress in a DSL line.

FIG. 2 shows an experimental setup for measuring noise at operator (CO)side of the line.

FIG. 3 shows an experimental setup for measuring noise at remote (CP)side of the line.

FIG. 4 shows a graph of noise as a function of frequency measured atboth sides of a line.

FIG. 5 shows a graph of estimated distance to a noise injection point asa function of frequency.

FIG. 6 shows a graph of estimated noise power at an ingress point as afunction of frequency.

FIG. 7 shows a graph of noise as a function of frequency measured atboth sides of a line.

FIG. 8 shows a graph of estimated distance to a noise injection point asa function of frequency.

FIG. 9 shows a graph of estimated noise power at an ingress point as afunction of frequency.

FIG. 10 shows a typical setting for the invention.

FIG. 11 shows a flow chart of a method embodying the invention.

FIG. 12 shows a flow chart of a method embodying the invention in a CPE.

FIG. 13 shows a flow chart of a method embodying the invention in aDSLAM.

FIG. 14 shows a flow chart of a method embodying the invention in an O&Mdevice.

DETAILED DESCRIPTION OF EMBODIMENTS

In this document, the terms “line” “transmission line” and “loop” willbe used interchangeably to denote the twisted copper pair used in DSLsystems to join the DSLAM and CPE.

Noise ingress is when noise enters a transmission line, noise ingresspoint is the point or location where noise enters the line.

FIG. 10 shows a typical setting for a DSL communication system

A central office 20 connects numerous customer premises sites 30 withtelecommunication transmission lines 100. At the customer premises, thelines 100 terminate at DSL modems 120 usually called CPEs (CustomerPremises Equipment). The CPEs are normally connected to various otherequipment 140, typically personal computers.

At the central office site 20, the lines 100 terminate at DSLAMs 110(Digital Subscriber Line Access Multiplexer). The DSLAMs are connectedto the internet 10, so as to provide internet connectivity to thecustomer equipment 140 via the lines 100 and the CPEs 120.

An operations and maintenance device (O&M device) 150 for operation andmaintenance of the DSL equipment and lines is connected to the DSLAMs.The device may be used to control various operational settings of theDSLAMs 110 as well as requesting the DSLAMs to carry out variousdiagnostic operations. It may contain a transmission line databaseholding information about the lines which terminate at the centraloffice 20.

The CPEs and DSLAMs may contain, inter alia, transmit and receivehardware and filter functionality, all of which may be configurable.

DSL Diagnostics

In DSLsystems, such as ADSL and VDSL2, physical cable faults in wirelinecommunication systems can be detected with Single-Ended Line Test(SELT), e.g. WO 2006/059175 A1 and Double-Ended Line Test methods (DELT)such as the Loop Diagnostics included in ADSL2 [G.992.3] and VDSL2[G.993.2] standards. Sometimes, SELT can also be used to locate theposition of the fault, e.g. interruption of the cable.

ADSL2 and VDSL2 are examples of FDM (frequency division multiplex)systems. In such systems, the CPE and DSLAM use disjoint frequency bandsfor their transmission. There is an upstream band for transmission fromthe CPE to the DSLAM and a downstream band for transmission from theDSLAM to the CPE. The up- and downstream bands may consist of multipledisjoint smaller bands.

Within the bands, data is normally transmitted by modulated carriertones at particular frequencies called subcarriers.

The bands used for transmission at an end of the line are calledtransmit bands with respect to that end of the line or with respect tothe equipment there, the bands used for reception are likewise calledreceive bands. Hence, at the CPE the transmit bands make up the upstreamand the receive bands make up the downstream, whereas at the DSLAM thetransmit bands make up the downstream and the receive bands make up theupstream.

Line noise in a receive band can be a problem. The achievable data ratefor a particular carrier frequency is limited by the SNR (Signal toNoise Ratio) at the receiving end, which depends on the transmit powerat the transmitting end, the line attenuation and the receive band noisefor that frequency.

The achievable data rate is commercially important, as a higher totaldata rate generally is charged higher. Also, strong noise can affect thestability of a DSL line, leading to customer complaints and thusincreased OPEX for the operator

DSL systems often have the possibility to measure receive band noiselevels per subcarrier. This measurement can be used to detect andidentify excessive crosstalk and certain other types of noise ingress onthe line, e.g. Radio Frequency Interference (RFI) from broadcaststations and some types of impulse noise.

ADSL2 and VDSL2 technologies have built-in standardized Loop Diagnosticfunctionality that among other things can measure receive bandQuiet-Line Noise (QLN), which is the near-end received power when thefar-end transmitter is silent, and the logarithmic channel transferfunction (HLOG) [G.992.3, G.993.2, G997.1]. These parameters are givenwith one value per used subcarrier in the configured band plan.

Improved DSL Noise Diagnostics

Noise levels in the transmit band generally do not affect the lineperformance. However, if QLN nevertheless could be measured also in thetransmit band, so that noise could be measured at both ends of the linefor the same frequency, this would enable improved diagnostics of noiseproblems.

In particular, the relationship between noise levels at both ends of theline for a particular frequency may be used to estimate a point alongthe line where noise enters the line, and also to estimate the noiselevel at that point.

Thus, in a DSL system, if noise power per subcarrier on a DSL line ismeasured by the DSLAM (xTU-O) and CPE (xTU-R) with overlap such that atleast some subcarriers are measured by both transceivers, this enablesimproved diagnostics.

By comparing the received power levels at both ends of the line, it ispossible to determine if the noise is closer to the operator's side orthe remote (customer's) side. If more information is available, such asloop length and/or cable attenuation (e.g. from SELT), a more precisedistance to the noise ingress point can be determined.

By locating the noise ingress point automatically, troubleshooting timecan be substantially decreased leading to large OPEX savings since theoperator will be able to tell e.g. whether the noise source is in thecustomer's home or a couple of blocks away. This will narrow down thesearch.

Even approximate knowledge of the location of noise entry may be veryuseful to rule out possible noise sources, and to decide on a locationwhere further work in the field is to be done. Field work is generallyquite expensive.

Theoretical Calculations

This section derives formulas allowing location of the ingress pointassuming that noise measurements can be performed for the samesubcarriers at both ends of the loop. The formulas are derived underidealized conditions such as homogeneous cables and perfect impedancematch at both ends of the loop but are still sufficiently accurate inseveral practical environments.

All calculations are done independently for each subcarrier meaning thatit is possible to locate up to as many narrowband noise sources as thereare subcarriers in the measured set.

FIG. 1 shows a schematic picture of noise ingress in a DSL line. Totalcable length is d, split into two segments with length d_(R) and d_(O).Noise PSD at the ingress point is N(f), resulting in N_(R)(f) on theremote side (left) and N_(O)(f) on the operator's side (right). Thecable transfer function (inverse of attenuation) is H(f), with H_(R)(f)and H_(O)(f) each for the two segments.

Thus FIG. 1 shows a scenario with a noise ingress point at distanced_(R) from the remote (customer premises) side. Assuming that distancesare given in meters, the noise PSD in dBm/Hz, and transfer function(inverse of attenuation) in dB, we have:

N _(R)(f)=N(f)+H _(R)(f)   (1)

N _(O)(f)=N(f)+H _(O)(f)   (2)

Here, it should be noted that H(f), H_(R)(f), and H_(O)(f) are normallynegative dB values since the copper loop is passive and does not amplifythe noise. Subtracting (1) from (2) gives:

N _(R)(f)−N _(O)(f)=H _(R)(f)−H _(O)(f),   (3)

which is independent of the noise PSD at the ingress point. If we don'thave any more information available, we could already say from the signof the left hand side of (3) whether the noise ingress point is(electrically) closer to the operator (O) or the remote (R) side sincethe right hand side corresponds to the difference in transfer function(dB) between the two segments in FIG. 1.

Equation (3) above can be rewritten by using the fact that the magnitudeof the transfer function of a telecommunication cable is proportional todistance for frequencies used in xDSL systems (4), (5), i.e.:

$\begin{matrix}{{{H_{R}(f)} = {\frac{d_{R}}{d_{R} + d_{0}}{H(f)}}},} & (4) \\{{H_{O}(f)} = {\frac{d_{O}}{d_{R} + d_{O}}{{H(f)}.}}} & (5)\end{matrix}$

Thus, we have:

${{{N_{R}(f)} - {N_{O}(f)}} = {{\frac{d_{R}}{d_{R} + d_{O}}{H(f)}} - {\frac{d_{O}}{d_{R} + d_{O}}{H(f)}}}},$

which after rearranging becomes:

$\begin{matrix}{{\left( {{N_{R}(f)} - {N_{O}(f)}} \right)\frac{d_{R} + d_{O}}{H(f)}} = {d_{R} - {d_{O}.}}} & (6)\end{matrix}$

This can be further rewritten as

$\begin{matrix}{{{\left( {{N_{R}(f)} - {N_{O}(f)}} \right)\frac{1}{H^{\prime}(f)}} = {d_{R} - d_{O}}},} & (7)\end{matrix}$

where H′(f) is the normalized transfer function (magnitude) in dB permeter for the used cable. If this property of the cable is known orassumed, we can with help of (7) tell the difference in length betweenthe two cable segments, i.e. how far from the centre the ingress pointis located. To get more useful information regarding the noise location,we would also need the loop length. This can for example be measuredwith SELT, estimated from measured attenuation values, or fetched fromdata bases describing the copper plant. By utilizing that d_(R)+d_(O)equals the loop length d in combination with (6), we get:

${{\left( {{N_{R}(f)} - {N_{O}(f)}} \right)\frac{d}{H(f)}} = {d_{R} - \left( {d - d_{R}} \right)}},$

which after simplification yields the distance from the remote end tothe noise ingress point:

$\begin{matrix}{\left. {d_{R} = {{\frac{1}{2}\left\lbrack {d + \left( {{N_{R}(f)} - {N_{O}(f)}} \right)} \right)}\frac{d}{H(f)}}} \right\rbrack.} & (8)\end{matrix}$

If the loop length d is not known, the result may be expressed insteadas a percentage of the loop length, which may still be useful.

Once the noise location is determined, we can also calculate the noisePSD N(f) at the ingress point, from equation (1):

N(f)=N _(R)(f)−H _(R)(f).

By rewriting, this yields the noise PSD as:

$\begin{matrix}{{N(f)} = {{N_{R}(f)} - {\frac{d_{R}}{d}{H(f)}}}} & (9)\end{matrix}$

Often, it is not necessary to know the transfer function H(f) for allfrequencies of interest; it is commonly known that the attenuation in dBfor many telecommunication cables (typically those with Polyethyleneinsulation) can be rather accurately estimated by a simple model (A, C=0in the cable model in section 7.2.1.3.2 in [G.993.2]) from a couple ofhundred kilohertz up to at least tens of MHz:

Ĥ(f)=d·k√{square root over (f)}, where k is a cable-dependent constant  (10)

In this case, it would be sufficient to measure the transfer function orthe attenuation at a single frequency, say 300 kHz or 1 MHz, in order tocalculate the cable constant k and the expressions for noise locationand noise power would become:

$\begin{matrix}{{{\hat{d}}_{R} = {\frac{1}{2}\left\lbrack {d + \frac{{N_{R}(f)} - {N_{O}(f)}}{k\sqrt{f}}} \right\rbrack}},} & (11) \\{{\hat{N}(f)} = {{N_{R}(f)} - {{d_{R} \cdot k}\sqrt{f}}}} & (12)\end{matrix}$

The calculations presented above are quite simple and should be ratherrobust. However, some problems that could occur when implementing thisin practice are:

Low accuracy on noise PSD measurements in DSLAM and/or CPE necessitatingcalibration of the hardware in order to get reliable noise PSD values.

Strong crosstalk noise (NEXT or FEXT), drowning the impulse noise or RFIingress at one or both sides of the loop. Of course, if the noise isdrowned by crosstalk at both sides, it could be argued that the noisesource is not interesting and will only marginally affect performance ofthe line.

However, even if only a low quality result is achieved, it may be highlyuseful. For example, if it can ruled out that the noise source islocated at the customer premises, there will be no need to book a visitto the customer's home and no need to ask the customer to look forpotential noise sources in her home.

When a noise source spans a frequency band so that the noise from thesame source is measured at several frequencies, an improved estimate ofthe position where the noise enters the line can be achieved by usingmore than one measurement. For example, a position estimate can be madefor each frequency, and a further estimate generated as the average ofthose estimates.

The invention may thus be embodied as a method in a system comprising aCPE 120, a transmission line 100, a DSLAM 110 and a device for operationand maintenance (O&M device) 150. The invention could also be embodiedas a method in the CPE, the DSLAM or the O&M device. Further theinvention could be embodied as a CPE, DSLAM or O&M device adapted tocarry out the corresponding method.

With reference to FIG. 11, a method embodying the invention is asfollows

In a step 1110 the noise level at a frequency or frequency band at thecustomer end of the line is measured by the CPE. The frequency orfrequency band is within a transmit band of the CPE or the DSLAM.

In a step 1120 the measurement result is communicated over the line tothe DSLAM

In a step 1130 the result is further communicated to the O&M device.

In a step 1140 the noise level is measured at the same frequency orfrequency band by the DSLAM at the central office end of the line.

In a step 1150 the result from step 1140 is also communicated to the O&Mdevice.

In a step 1160 a position where noise enters the line is estimated independence of the relationship between the noise levels. The estimate ismade in the O&M device.

If the O&M device is integrated into the DSLAM, steps 1130 and 1150 arenot needed, or take place within the DSLAM.

With reference to FIG. 12, a method embodying the invention in a CPE isas follows.

In a step 1210, a noise measurement is made at a frequency or frequencyband which is within a transmit band of the CPE.

Optionally, in a step 1220, the result of the measurement iscommunicated over the line.

(When not performing the step 1220, the measurement may for example becommunicated elsewhere, or in another way, or not communicated, e.g. ifthe measurement is only to be stored, or if it is to be used in theCPE.)

With reference to FIG. 13, a method embodying the invention in a DSLAMis as follows

In a step 1310, a noise measurement is made at a frequency or frequencyband which is within a transmit band of the DSLAM.

With reference to FIG. 14, a method embodying the invention in an O&Mdevices is as follows.

In a step 1410, a result of a noise measurement at a frequency orfrequency band at the CPE end of the line made by the CPE is receivedvia the line. The frequency or frequency band is within a transmit bandof the CPE or the DSLAM.

In a step 1420, a result of a noise measurement at the same frequency orfrequency band at the DSLAM end of the line made by the DSLAM isreceived.

In a step 1430, an estimate of the position where noise enters the lineis made by the O&M device.

In an optional step 1440 an estimate is made of the noise level at thepoint where noise enters the line.

The methods described above may be embodied in a system according toFIG. 10

The O&M device 150 is typically a workstation or similar computer. Therewill be program code in the device for receiving a noise measurementresult from the CPE. The result is typically sent from the CPE 120 tothe DSLAM 110 over the line and then further to the device 150. Therewill also be code for receiving a measurement result from the DSLAM 110,and program code for generating an estimate of a position where noiseenters the transmission line in dependence of the relationship betweenthe first and the second noise level measurement results. There may alsooptionally be program code for estimating the noise level at the pointwhere noise enters the line.

In another embodiment the functions of the device 150 are integratedinto the DSLAM 110.

The CPE 120 may have program code for, or otherwise be configured tomake noise level measurements in one or more of its transmit bands. Anyfilter through which the measurement is made should be properly applied,so that measurement is made in a pass band of the filter. There may alsobe program code for sending measurements results over the line.

The DSLAM 110 may have program code for, or otherwise be configured tomake noise level measurements in one or more of its transmit bands. Anyfilter through which the measurement is made should be properly applied,so that measurement is made in a pass band of the filter. There may alsobe program code for receiving measurement results from the CPE 120 aswell as code for sending measurement results to the O&M device 150. Theresults may be from measurement in the DSLAM 110 or received from theCPE 120.

When performing Loop Diagnostics with hardware supporting ADSL2(+) AnnexM subcarriers 32-64 are reported for both ends of the loop but the high-and lowpass filters used by the modems to separate the transmit andreceive bands attenuate the measurements too much at one or both ends ofthe loop. In other words, the overlap in frequency domain is only forthe filter edges and not for the filter passbands, which means that itis not usable for practical purposes.

Outside the receiver passbands, the reported results reflect thereceiver noise rather than the quiet line noise.

Thus, even though tones which are part of the transmit band may bereported, the measurement is made through the receive band filter, andthe transmit band tones are therefore outside of the filter pass band.In general, the measured frequencies should fall within the pass band ofthe filter through which the measurement was made.

When implementing the above methods in a CPE and/or DSLAM, the followingapproaches may be taken in order to reuse existing functionality to asuitable degree

-   -   Implement a modified Loop Diagnostics mechanism that measures        QLN at both ends of the loop for all used subcarriers    -   Complement the QLN from loop diagnostics with Single-Ended Line        test (SELT) noise measurements adapted to measure in transmit        bands. e.g. use DSLAM SELT adapted to measure noise PSD on the        downstream frequencies (where Loop Diagnostics gives only CPE        noise measurements).    -   Use adapted SELT noise measurements at both ends of the loop,        i.e. both DSLAM SELT and CPE SELT.

The first solution is, at least in theory, straightforward but requireschanges to existing standards, possibly leading to interoperabilityproblems. Also, not all existing hardware can be adapted to measure ontransmit bands. For example, typical ADSL2+ DSLAMs can only be made tomeasure (receive) on the first 64 (upstream) tones while they can bemade to transmit on 512 tones. This situation improves with VDSL2 sincea VDSL2 modem must support several different band plans, meaning thatthe hardware has to be more flexible and allow reception over widerfrequency bands.

The second solution is definitely feasible but also suffers from thesame hardware limitation problem as the first solution. Further, withcurrent standards, it will only support noise ingress localization inthe downstream bands since no noise measurements will be available forthe upstream bands in the CPE.

The third solution allows noise localization on all subcarriers jointlysupported by the transceivers but requires adapted SELT measurementsboth at the Central Office (CO) side and at the Customer Premises (CP)side. It should be noted that SELT is currently not yet standardizedalthough a draft first version of the G.linetest (G.996.2) standard wasconsented by ITU-T SG15/Q4 in December 2008. Further, in the firstdraft, it is still not agreed on how to manage CPE SELT and how to getthe measurement results communicated to the DSLAM side. However, it isexpected that the standard will continue to evolve in the near future.

Of the three solutions presented above, number two has the advantagethat it can be utilized without modifications to existing CPEs thatsupports Loop Diagnostics. Of course, the DSLAM should be adapted tosupport SELT noise measurements for the downstream (transmit) bands.

For accurate location of narrowband noise sources the noise sourceshould be dominant at both ends of the loop, i.e. the majority of thenoise measured for a certain subcarrier must come from the same sourceat both ends of the loop.

However, should that not be the case, even a less accurate estimate maybe highly valuable.

NUMERICAL EXAMPLES

The following section will show numerical examples from laboratoryexperiments in order to verify the potential of the invention.

FIG. 2 shows an experimental setup for measuring noise at operator (CO)side of the line.

FIG. 3 shows an experimental setup for measuring noise at remote (CP)side of the line.

To be able to show examples utilizing the whole frequency band up to 17MHz, laboratory experiments were performed. A VDSL2 DSLAM adapted tomeasure noise over the full band was used to measure the noise on bothsides of a loop consisting of 200+500 meter of 0.4 mm telephone cable.Firstly, the setup in FIG. 2 was used to measure the noise on the wholeband for the CO side of the loop and secondly, since no suitably adaptedCPE was available, the DSLAM was moved to the other end of the loopaccording to FIG. 3 in order to measure on the CP side. The latter isunrealistic in a real scenario but was done to visualize the greatpotential the noise localization method will have if the standards areevolved as discussed earlier.

In a first example, Additive White Gaussian Noise (AWGN) with a flat PSDlevel of −90 dBm/Hz has been injected 500 m from the CO side of a loopwith a total length of 700 m. The noise injection point is thus located200 m from the CP side of the loop. FIG. 4 shows the received noise atboth sides of the loop (NO(f) and NR(f)). Since the attenuation ishigher for the longer side of the loop the received noise is weaker atthe CO side than on the CP side.

In this scenario the cable attenuation at 1 MHz was known to be 20.5dB/km. With this information and the fact that the cable was 700 m long,it is possible to use Equation (11) to estimate the location of thenoise injection point. The result from this calculation, i.e. theestimated distance from each end of the loop to the noise injectionpoint, is shown in FIG. 5. As can be seen in the figure, the noiselocation is estimated to be very close to the true location of 200 mfrom the CP side and 500 m from the CO side. The error at highfrequencies is because the noise was too much attenuated at the CO sidefor these frequencies so the injected noise was no longer dominant overthe background noise when measured at the CO side. This can be seen inFIG. 4 where the noise graph flattens out at high frequencies. Theripple at low frequencies is caused by the noise injector used in theexperiment.

Finally Equation (12) is used to calculate the PSD of the noise at theinjection point as shown in FIG. 6.

The noise is supposed to be flat for the whole spectrum but once againthe effect of the weak received noise signal at the CO side can be seenon high frequencies. The offset from −90 dBm/Hz at lower frequencies islikely a combination of measurement errors in the DSLAM, impedancemismatch and imperfect calibration of the noise generator and noiseinjection box.

A perhaps more common type of noise that could be found on a DSL loop isRFI. The same loop as in the first example has therefore been used againbut this time two RFI peaks with a PSD level of −60 dBm/Hz, at 10 MHzand 15 MHz respectively have been injected 500 m from the CO side.

FIG. 7 shows the measured noise on each side of the loop. Two RFI peaksat 10 and 15 MHz have been injected 500 m from the CO side. Themeasurements are marked with a circle for the CO side and an asteriskfor the CPE side.

FIG. 8 shows the estimation of the RFI noise location. The peaks at 10and 15 MHz are correctly estimated at about 500 m from the CO side.

Since it is only the RFI peaks that are injected at 500 m, the noise atall other frequencies will be located in the middle of the loop i.e. at350 m (assuming that the background noise has the same PSD at both sidesof the loop). The deviation at low frequencies is probably caused bysome other noise source in the laboratory, closer to the CP side of theloop. The location of the RFI peaks is correctly estimated at 500 m fromthe CO side.

FIG. 9 shows the estimated noise power at the source. The power at thenoise source has been calculated from Equation (12).

The graph shows a slope corresponding to the cable attenuation since itis assumed that the background noise origins from a source close to themiddle of the cable. The RFI peaks are well estimated at approximately−60 dBm/Hz.

Since the algorithm assumes that all noise origins from the noiseinjection point, the background noise is also used in the calculationsof the noise power at the source. The background noise is thereforecompensated for the attenuation from the end point of the loop to thenoise injection point and hence the estimated noise graph at the noisesource shows a slope corresponding to the attenuation. In this exampleit can be seen in FIG. 7 that the noise PSD is low except for the RFIpeaks and thus it is only the peaks in FIG. 9 that are of interest.These are correctly estimated to have a PSD of approximately −60 dBm/Hz.

A very crude location estimate can be obtained already from measurementof the noise PSDs at both ends of a loop. Refined location estimates canbe achieved by, in addition to the noise measurements, also utilizingloop length and attenuation information. This type of information can beestimated with good accuracy from SELT, DELT or similar methods.

1. A method for estimating a position where noise enters atelecommunications transmission line the line having a firstcommunications equipment connected to a first end of the line and asecond communications equipment connected to a second, opposite end ofthe telecommunications transmission line, the method comprising thefollowing steps: receiving, via the telecommunications transmissionline, a first result of a measurement of a noise level at the first endof the telecommunications transmission line made by the firstcommunications equipment; receiving a second result of a measurement ofa noise level at the second end of the telecommunications transmissionline made by the second communications equipment; and generating anestimate of the position where the noise enters the telecommunicationstransmission line based on the relationship between the first and thesecond noise level measurement results where the first and second noiselevel measurements are made at the same frequency or frequency band andthe frequency or frequency band falls within a transmit band of at leastone of the first and second communications equipment.
 2. The methodaccording to claim 1 wherein a position estimate is generated as$\left. {d_{R} = {{\frac{1}{2}\left\lbrack {d + \left( {{N_{R}(f)} - {N_{O}(f)}} \right)} \right)}\frac{d}{H(f)}}} \right\rbrack$where d_(R) is a distance from the first end to the estimated position,d is a length of the telecommunications transmission line from the firstend to the second end, N_(R)(f) is the first result of the measurementof the noise level at the first end at frequency f, N_(O)(f) is thesecond result of the measurement of the noise level at the second end atsaid frequency f, and H(f) is the magnitude of the telecommunicationstransmission line's transfer function from the first end to the secondend at said frequency f.
 3. The method according to claim 2, wherein theposition estimate is generated for different frequencies and a furtherposition estimate is generated as an average of the position estimatesgenerated for the different frequencies.
 4. The method according toclaim 1, further comprising the step of generating an estimate of anoise power spectral distribution at the position where the noise entersthe telecommunications transmission line, based on the relationshipbetween the first and the second noise level measurement results.
 5. Themethod according to claim 4 wherein the estimate of the noise powerspectral distribution (PSD) is generated as${N(f)} = {{N_{R}(f)} - {\frac{d_{R}}{d}{H(f)}}}$
 6. The methodaccording to claim 1, wherein the first communications equipment is afirst frequency division multiplex (FDM) communications equipment whichperforms functions similar to an xDSL consumer premises equipment (CPE).7. The method according to claim 1, wherein the second communicationsequipment is a second FDM communications equipment which performsfunctions similar to an xDSL digital subscriber line access multiplexer(DSLAM).
 8. The method according to claim 1, wherein thetelecommunications transmission line includes a copper pair wire.
 9. Adevice for estimating a position where noise enters a telecommunicationstransmission line the telecommunications transmission line having afirst communications equipment connected to a first end of the line anda second communications equipment connected to a second end, opposite tothe first end of the line, the device comprising: means for receiving,via the telecommunications transmission line, a first result of ameasurement of a noise level at the first end of the telecommunicationstransmission line made by the first communications equipment; means forreceiving a second result of a measurement of a noise level at thesecond end of the telecommunications transmission line made by thesecond communications equipment; and means for generating an estimate ofthe position where the noise enters the telecommunications transmissionline based on the relationship between the first and the second noiselevel measurement results where the first and second noise levelmeasurements are made at the same frequency or frequency band and thefrequency or frequency band falls within a transmit band of at least onof the first and second communications equipment.
 10. The deviceaccording to claim 9, further having means for generating an estimate ofa noise power spectral distribution at the position where the noiseenters the telecommunications transmission line, based on therelationship between the first and the second noise level measurementresults.
 11. The device according to claim 9, wherein thetelecommunications transmission line includes a copper pair wire.
 12. Acommunications equipment having a plurality of transmit bands andadapted for use with the method of claim 1, wherein the communicationsequipment is adapted to measure quiet line noise (QLN) in at least oneof the transmit bands.
 13. The communications equipment according toclaim 12 being adapted to measure the QLN at a plurality of frequenciesused for reception or transmission of data.
 14. The communicationsequipment according to claim 12, wherein the communications equipment isa consumer premises equipment (CPE) configured for use in a frequencydivision multiplex (FDM) environment.
 15. The communications equipmentaccording to claim 12, wherein the communications equipment is afrequency division multiplex equipment which performs functions similarto an xDSL digital subscriber line access multiplexer (DSLAM) in afrequency division multiplex (FDM) environment.
 16. The equipmentaccording to claim 12, wherein the communications equipment is adaptedto make said transmit band QLN measurement in a passband of a filterthrough which the measurement is made.
 17. The method of claim 1,further comprising: measuring quiet line noise (QLN) in at least one ofa plurality of transmit bands.
 18. The method according to claim 17wherein the QLN is measured at a plurality of frequencies used forreception or transmission of data.
 19. The method according to claim 17,wherein said transmit band QLN measurement is made in a passband of afilter through which the measurement is made.
 20. A method of estimatinga position where noise enters a telecommunications transmission line,the method comprising the steps of: measuring a first noise level at afirst frequency or frequency band at a first end of the transmissionline by a consumer premises equipment (CPE); communicating the measuredfirst noise level over the telecommunications transmission line to anequipment at a second end of the transmission line, opposite the firstend; measuring a second noise level at the first frequency or frequencyband at the second end by the equipment; and estimating, based on therelationship between the first and second noise level, the positionwhere the noise enters the transmission line wherein the first frequencyor frequency band falls within a transmit band of at least one of theCPE and the equipment.